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Circuit simulation explained
What happens behind the Run button, in plain language.
DC steady state (Run)
When you press Run, the simulator solves the whole circuit at once for a steady state. Capacitors are treated as open circuits, because after a long time no current flows through a charged capacitor. You then see voltage, current and power for the selected part.
Watching a capacitor charge (Start Transient)
If your circuit contains a capacitor, Start Transient appears. It steps time forward so you can watch the voltage rise. The speed of an RC circuit is set by its time constant:
τ = R × C
After one τ the capacitor has reached about 63% of the supply, and after about five τ it is within 1% of final. For example, 10 kΩ and 100 µF give τ = 10,000 × 0.0001 = 1 second, so it is nearly full after roughly five seconds.
Parts and their behaviour
- LED: colour sets the forward voltage (red about 1.9 V up to white about 3.2 V). It needs a current-limiting resistor.
- Diode: conducts in one direction only; the practical “Diode rectifier check” shows this.
- NPN transistor: used as a switch — a small base current lets a larger collector current flow.
- Meter: tap two holes to read the voltage between them.
Warnings that teach
The simulator tells you about shorts, open circuits and LEDs without a current-limit resistor — the same faults that cause trouble on a real bench.
Op-amp basics
The LM741 practicals illustrate two rules. With negative feedback the two inputs sit at nearly the same voltage (a “virtual short”). Without feedback the op-amp acts as a comparator and its output saturates close to a supply rail.
- Inverting amplifier:
gain = −Rf ÷ Rin - Non-inverting amplifier:
gain = 1 + Rf ÷ Rg